In recent years, there has been a great increase in research activities relating to organic catalysis, especially chiral organic catalysis. Certain simple organic molecules (organocatalysts) are capable of effectively catalyzing a variety of fundamentally important transformations, leading to highly enantioselective products. Unlike organometallic catalysts, organocatalysts do not involve metals, giving them greater applicability in pharmaceuticals industry. However, organocatalysts still have some disadvantages, such as the requirement for high catalyst loading, and the difficulty in catalyst separation from the product stream. Solid supported catalysts have the potential to offer a solution to these disadvantages.
Polymer- or silica-supported imidazolidin-4-one catalysts have been reported before. However, there are no previous reports of imidazolidin-4-ones supported on mesocellular siliceous foam (MCF), and no previous reports of asymmetric Friedel-Crafts alkylation catalyzed by heterogenized catalysts.
A recyclable organocatalyst of high efficiency and enantioselectivity (for example imidazolidin-4-one) supported on MCF, for Friedel-Crafts alkylation and Diels-Alder cycloaddition, would be desirable. MCF is a novel mesoporous material with unique advantages as a solid support for catalysts. Compared to polymers, this silica support does not have swelling problems, and is more thermally robust. Also, the surface of silica may be easily modified to manipulate the microenvironment of the supported catalyst. Templated by oil-in-water microemulsions, MCF has a high surface area of commonly 500-800 m2/g, and a 3-dimensional pore structure with ultralarge cell-like pores (commonly 23-42 nm) that are connected by windows (commonly 9-22 nm). Such a pore structure may prevent steric issues associated with the immobilization of bulky compounds, and may minimize any diffusion limitation of large substrates.
Imidazolidin-4-one 1 (FIG. 1) is a chiral organocatalyst which has been used as an efficient catalyst for a variety of highly enantioselective reactions. It also has great application potential in industry. Therefore, heterogenization of this organocatalyst would be very attractive. Imidazolidin-4-one supported on polymer and silica gel was recently studied for cycloaddition reactions. However, the reported chemical efficiency of these heterogenized catalysts was much lower than the homogeneous catalyst.
It would therefore be desirable to produce a heterogeneous catalyst of comparable chemical and stereochemical efficiency to unsupported imidazolidin-4-one.